In the preparation of carbon artifacts such as graphite electrodes, a carbonaceous filler such as petroleum coke is admixed with a coal tar pitch binder and then formed, carbonized, and graphitized to produce a graphite product. For maximum product strength, it is important that the coal tar pitch binder give a good yield of carbon after carbonization. The presence of relatively high amounts of infusible carbon solids i.e. fine particles, generally called Q.I. (Quinoline Insoluble), is desirable for an effective binder in order to increase coking yield and to provide a source of fine carbon particles which also improve graphite artifact strength. Commercial coal tar binder pitches usually contain about 8-20% by weight Q.I. mainly in the form of small (micron) size spherulitic carbon particles. These particles, which are called natural Q.I., are generated during the preparation of the tar precursors used to produce the binder pitch. The Q.I. in pitches can also contain larger carbonaceous particles called cenospheres, carbonized coal particles, and inorganic ash. These components also originate in the preparation of the tar precursor and are generally not beneficial for use of the pitch as a binder. An additional form of Q.I. called secondary Q.I. or mesophase can be formed by heat treatment during the conversion of tar to pitch.
Very often, in order to increase strength, the carbon artifact is impregnated with molten pitch after baking, but before graphitization. The molten pitch impregnant fills the pores generated during the initial baking of the carbon article and increases final strength and density. In contrast to the requirements for binder pitch, an impregnant pitch should have very low or preferably zero amounts of solids (Q.I.). The presence of solid particles which are not miscible with the molten pitch would block the pores of the carbon article and prevent full impregnation of the pitch into the artifact.
It is presently difficult to produce impregnating pitches which are solids-free, i.e. Q.I. free. Conventional filtration or centrifugation of precursor coal tars can be used to remove the Q.I. particles prior to conversion to pitch. However, these operations are costly since they are batch operations and must be done at high temperatures. Additionally, the Q.I. particles must be separated from the solids-free tar and then disposed of. There is currently no domestic, i.e. United States source of a solids-free coal tar impregnating pitch. Batch processes have been developed in Japan for removal of Q.I. from coal tars to produce solids-free impregnating pitches (U.S. Pat. No 4,127,472) which involve treatment of the tar with an anti-solvent to settle the Q.I., followed by separation of the Q.I. by filtration or centrifugation. The separated Q.I. must then be disposed of. Japan published patent application 1(1989)-305,640 discloses the use of membrane filters to remove Q.I. solids from coal tar and coal tar pitch in a batch type procedure.
There is also difficulty obtaining high Q.I. content tars which are suitable for binder pitches. With increasing environmental controls, the coking operations used to produce the tars have been reduced in severity with a resulting reduction in the Q.I. levels in the tars. The derived pitches are, therefore, low in Q.I. and lead to reduced strength in graphite products when used as binder pitch. In Europe, Q.I. levels of binder pitch are generally below the minimum desired level of 8%. In order to increase the Q.I. content, processes have been developed in which artificial carbon fines are added back to the tar or pitch (U.S. Pat. No. 4,177,132).
For these reasons, it would be very advantageous to have a continuous process which could produce, at the same time, 0% Q.I. tars for impregnating pitches, and high Q.I. tars for binder pitches.
Over the last decade or so, an advanced form of ceramic membrane technology has become commercialized. This technology involves the use of ceramic monoliths, known as cross-flow filters, whose channel walls contain carefully controlled pore sizes. Pore sizes can be varied from somewhat above one micron down to 50 Angstroms.
These membranes operate in a fundamentally different manner from conventional dead-end filters. Instead of depositing the solids on a filter medium as occurs with dead-end filters, the feed stream flows across the surface of the membrane and the solids stay suspended in the liquid. The permeate or filtrate passes through the membrane and is collected.